Compounding ingredients for rubber can be conveniently packaged in bags made from thermoplastic polymers or paper while bales of unvulcanized rubber are often packaged by wrapping them with paper or with thermoplastic film, the latter being conventionally known as bale wrap. Polyethylene is usually used for these purposes because of its plasticity and antiblock properties and because it can be obtained at a relatively low cost. Indeed, polyethylene is the most widely used polymer for the manufacture of plastic films with over 4.4 billion pounds of film having been manufactured in the United States in 1979. A good portion of this film was used as shipping sacks, packaging films, pallet stretch wrap and shrink wrap.
However, the use of polyethylene, as well as some other thermoplastics, for rubber compounding bags or for bale wrap is not truly acceptable because of the incompatability of such thermoplastic materials with rubber compounds. The relatively high melting temperature and the fact that polyethylene is not crosslinkable by conventional curing mediums can lead to severe defects in rubber compounds when polyethylene is incorporated in the mix. Another significant disadvantage of the use of polyethylene is that it does not disperse sufficiently in the matrix when it is mixed with unvulcanized rubber. This leaves minute areas within the matrix of the rubber when cured, which are themselves not cured and are subject to degradation.
Thus, polyethylene, when incorporated into a compounding formulation through the use of compounding bags, bale wrap or shipping sacks, can be the source of several problems leading to potential batch or product rejects. The use of polyethylene for mixes where the drop temperatures are too low (ca. 250.degree. F.) for standard film grade polyethylene to adequately melt or disperse is of particular concern. The result is a defective tire, hose, belt, gasket or whatever rubber product is being manufactured.
One example of such a problem would be the buildup of gases at the undispersed polyethylene site in a tire which could lead to ply or tread separation. Another example of such problems is in connection with steam hoses where polyethylene voids can lead to blisters in the finished product. Polyethylene voids are also of concern in finished products such as belts and gaskets. In cases where compounding temperatures are high enough to melt and disperse the polyethylene, the polyethylene acts as a phase- changing, poorly compatible oil or liquid.
Various materials have been proposed as a substitute for polyethylene in such utilities, as disclosed, for example, in U.S. Pat. Nos. 4,110,500; 4,112,158; and 4,248,348 as a result of the industry's attempt to find an acceptable packaging material.